The measurement of blood pressure within the human heart provides critical information regarding the organ's function. Many methods and techniques have been developed to give physicians with the ability to monitor heart function to properly diagnose and treat various diseases and medical conditions. For example, a sensor or transducer placed within the chambers of the heart can be used to record variations in blood pressure based on physical changes to a mechanical element within the sensor. This information is then transferred from the sensor to external device that is capable of translating the data from the sensor into a measurable value that can be displayed. The drawback of this type of sensor is that there must be a physical connection between the sensor and the external device, thus limiting its use to acute settings.
Many types of wireless sensors have been proposed that would allow implantation of the device into the body and then through the appropriate coupling means, pressure reading can be made over longer periods of interest. The primary limitation to these type of sensors are the fabrication methods used to manufacture them do not provide sufficient miniaturization to allow them to be introduced and implanted into the heart using non-surgical, catheter based techniques.
For a sensor that is designated to be placed within the heart, additional considerations are required. A feature must be provided that will allow the sensor to be permanently secured to the wall of the heart in a manner that will exclude any possibility of movement of the sensor or displacement out of the heart. Finally, an implantable sensor of this type must be assembled using the materials and fabrications methods that ensure appropriate biocompatibility and long term mechanical and electrical durability.
One method of manufacturing a sensor capable of measuring pressure is to use a capacitor that is assembled such that the capacitive plates will deform as a result of exposure to externally applied stress. This deformation will result in a change in the capacitance that will be proportional to the applied stress. Various patents describe the fabrication and use of capacitor based pressure sensors. The primary limitation of many of these inventions is that the techniques used to fabricate the sensors do not lend themselves to the miniaturization necessary for it to be configured as an implantable medical device.
The fabrications methodologies that have been developed in the field of Micro-Electro-Mechanical Systems, however, do specifically provide the means for assembling miniaturized sensors capable of measuring a variety of properties including pressure. MEMS devices as described in these patents traditionally use silicon as a substrate for construction of miniature electrical or mechanical structures. The resulting sensors are inherently rigid severely limiting the ability to manipulate them into temporarily small packages that would provide the means for non-surgical implantation into the human body.
A number of patents detail pressure sensors (some capacitive in nature, some manufactured using MEMS based technology) that are specifically designed for implantation into the human body. These sensors suffer from many of the limitations already mentioned with the additional concerns that they require either the addition of a power source to operate the device or the need for a physical connection to a device capable of translating the sensor output into a meaningful display of a physiologic parameter.
To overcome these two problems (power and physical connection), the concept of a externally modulated LC circuit has been applied to development of implantable pressure sensors. Of a number of patents that describe a sensor design of this nature, Chubbuck, U.S. Pat. No. 6,113,553 is a representative example. The Chubbuck patent demonstrates how a combination of a pressure sensitive capacitor placed in series with an inductor coil provides the basis of a wireless, un-powered pressure sensor that is suitable for implantation into the human body. Construction of an LC circuit in which variations of resonant frequency correlate to changes in measured pressure and which these variations can be detected remotely through the use of electromagnetic coupling are further described in Allen et al., U.S. Pat. No. 6,111,520, incorporated herein by reference.
The device embodied by the Chubbuck patent is manufactured using conventional techniques, thus requiring surgical implantation and thus limiting its applicability to areas that are easily accessible to surgery (i.e., skull).
Thus, the need exists for a biocompatible, wireless, un-powered pressure sensor that for the purposes of introduction and delivery within the human heart can be manipulated into a smaller shape and size by rolling or folding it into a cylindrical form and loaded into a small diameter catheter. Then upon positioning the catheter within the desired chamber of the heart, the sensor can be deployed and through the use super-elastic alloy components in the form of anchors or hooks secured to the interior wall of the heart.